Motor vehicles are supported by inflatable tires having a desired inflation pressure. Improper inflation of a tire can lead to poor gas mileage and increased tire wear. Maintaining tire inflation within an acceptable range can alleviate these issues. To do so however, requires frequent measuring of tire pressure and evaluation of whether that pressure lies within an acceptable range. Moreover, tire pressure will also vary according to temperature and vehicle load.
Systems that automatically monitor pressure and provide an indication to the vehicle operator when the fluid (typically air) within the pressure vessel (e.g., the tire) reaches a condition of improper inflation typically include a pressure sensor in communication with an external receiver capable of interfacing with the vehicle operator. These systems are typically a collection of remote electronic circuits for both sensing the tire pressure and communicating the pressure information to a separate receiver, which in-turn interfaces with the vehicle operator. The actual pressure sensor may be located external to the tire, for example on or in the valve stem of the wheel. The sensor may also be located directly within the mounted wheel/tire assembly. Because these direct system sensors are located in remote areas of the vehicle, these direct systems are required to operate under their own power (e.g., battery power).
The communication link between the sensor and the external receiver may be wireless, with radio frequency signals and/or infrared or optical signals being the most common forms. Although the utility of wireless communication in these direct systems is severely limited due to reliance on battery power, these sensors can physically measure the tire pressure and transmit the tire pressure, temperature, battery level, sensor ID number, and even location information out of the rotating tire by RF signal. The direct system offers accuracy and fast response compared to systems employing an ABS wheel speed sensor to detect tire pressure. The disadvantage of the direct system is that the cost is relative high.
Because the sensor is installed inside the tire, it is not easy to physically touch the sensor in a direct system. There is a need in some instances to communicate with the sensor even though it is located within the tire, for example, to drive the sensor into a test mode, to force the sensor to transmit for the test mode, to drive the sensor into a sleep mode (to stop transmit and save battery power), to wake the sensor from sleep mode, telling the sensor it's relative location (right front or left rear), and to pass calibration parameters to the sensor, etc. A common method to talk to the sensor employs using low frequency signals, 125 kHz. For receiving the low frequency signals, a coil is used as an antenna. Because the low frequency field strength is very low, the required efficiency of the coil is relatively high resulting in a higher cost for such a coil. Current coil designs employ a surface mounted (SMD) small ferrite core coil to obtain the required efficiency. These surface mounted inductors cost in excess of $0.20 per unit in large volumes.
While employing a SMD ferrite core coil serves its antenna function, it proves to be costly and increases the componentry on the printed circuit board on which it is employed. Accordingly, a tire pressure sensing and transmit/receiving assembly having a less costly antenna to receive low frequency signals involving less componentry would be of great benefit.